<?xml version='1.0' encoding='utf-8'?>
<FilingSummary>
  <Version>3.26.1</Version>
  <ProcessingTime/>
  <ReportFormat>html</ReportFormat>
  <ContextCount>234</ContextCount>
  <ElementCount>255</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>true</FootnotesReported>
  <SegmentCount>52</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>6</UnitCount>
  <MyReports>
    <Report instance="ctmx-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R1.htm</HtmlFileName>
      <LongName>75000 - Document - Document and Entity Information</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20260630/taxonomy/role/DocumentDocumentAndEntityInformation</Role>
      <ShortName>Document and Entity Information</ShortName>
      <MenuCategory>Cover</MenuCategory>
      <Position>1</Position>
    </Report>
    <Report instance="ctmx-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>75010 - Statement - CONDENSED BALANCE SHEETS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20260630/taxonomy/role/Role_StatementCONDENSEDBALANCESHEETS</Role>
      <ShortName>CONDENSED BALANCE SHEETS</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>2</Position>
    </Report>
    <Report instance="ctmx-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R3.htm</HtmlFileName>
      <LongName>75020 - Statement - CONDENSED STATEMENTS OF OPERATIONS AND COMPREHENSIVE INCOME (LOSS)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20260630/taxonomy/role/StatementCondensedStatementsOfOperationsAndComprehensiveIncomeLoss</Role>
      <ShortName>CONDENSED STATEMENTS OF OPERATIONS AND COMPREHENSIVE INCOME (LOSS)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>3</Position>
    </Report>
    <Report instance="ctmx-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R4.htm</HtmlFileName>
      <LongName>75030 - Statement - CONDENSED STATEMENTS OF STOCKHOLDERS' EQUITY (DEFICIT)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20260630/taxonomy/role/StatementCondensedStatementsOfStockholdersEquityDeficit</Role>
      <ShortName>CONDENSED STATEMENTS OF STOCKHOLDERS' EQUITY (DEFICIT)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>4</Position>
    </Report>
    <Report instance="ctmx-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R5.htm</HtmlFileName>
      <LongName>75040 - Statement - CONDENSED STATEMENTS OF CASH FLOWS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20260630/taxonomy/role/Role_StatementCONDENSEDSTATEMENTSOFCASHFLOWS</Role>
      <ShortName>CONDENSED STATEMENTS OF CASH FLOWS</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>5</Position>
    </Report>
    <Report instance="ctmx-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R6.htm</HtmlFileName>
      <LongName>995410 - Disclosure - Pay vs Performance Disclosure</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://xbrl.sec.gov/ecd/role/PvpDisclosure</Role>
      <ShortName>Pay vs Performance Disclosure</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>6</Position>
    </Report>
    <Report instance="ctmx-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R7.htm</HtmlFileName>
      <LongName>995445 - Disclosure - Insider Trading Arrangements</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://xbrl.sec.gov/ecd/role/InsiderTradingArrangements</Role>
      <ShortName>Insider Trading Arrangements</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>7</Position>
    </Report>
    <Report instance="ctmx-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R8.htm</HtmlFileName>
      <LongName>995455 - Disclosure - Description of the Business</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20260630/taxonomy/role/Role_DisclosureDescriptionOfBusiness</Role>
      <ShortName>Description of the Business</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>8</Position>
    </Report>
    <Report instance="ctmx-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R9.htm</HtmlFileName>
      <LongName>995465 - Disclosure - Basis of Presentation and Summary of Significant Accounting Policies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20260630/taxonomy/role/Role_DisclosureBasisOfPresentationAndSummaryOfSignificantAccountingPolicies</Role>
      <ShortName>Basis of Presentation and Summary of Significant Accounting Policies</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>9</Position>
    </Report>
    <Report instance="ctmx-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R10.htm</HtmlFileName>
      <LongName>995475 - Disclosure - Net Income (Loss) Per Share</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20260630/taxonomy/role/DisclosureNetIncomeLossPerShare</Role>
      <ShortName>Net Income (Loss) Per Share</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>10</Position>
    </Report>
    <Report instance="ctmx-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R11.htm</HtmlFileName>
      <LongName>995485 - Disclosure - Fair Value Measurements and Investments</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20260630/taxonomy/role/DisclosureFairValueMeasurementsAndInvestments</Role>
      <ShortName>Fair Value Measurements and Investments</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>11</Position>
    </Report>
    <Report instance="ctmx-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R12.htm</HtmlFileName>
      <LongName>995495 - Disclosure - Accrued Liabilities</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20260630/taxonomy/role/Role_DisclosureAccruedLiabilities</Role>
      <ShortName>Accrued Liabilities</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>12</Position>
    </Report>
    <Report instance="ctmx-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R13.htm</HtmlFileName>
      <LongName>995505 - Disclosure - Collaboration and License Agreements</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20260630/taxonomy/role/DisclosureCollaborationAndLicenseAgreements</Role>
      <ShortName>Collaboration and License Agreements</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>13</Position>
    </Report>
    <Report instance="ctmx-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R14.htm</HtmlFileName>
      <LongName>995515 - Disclosure - License Agreement</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20260630/taxonomy/role/DisclosureLicenseAgreement</Role>
      <ShortName>License Agreement</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>14</Position>
    </Report>
    <Report instance="ctmx-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R15.htm</HtmlFileName>
      <LongName>995525 - Disclosure - Common Stock</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20260630/taxonomy/role/Role_DisclosureCommonStock</Role>
      <ShortName>Common Stock</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>15</Position>
    </Report>
    <Report instance="ctmx-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R16.htm</HtmlFileName>
      <LongName>995535 - Disclosure - Stock-Based Compensation</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20260630/taxonomy/role/Role_DisclosureStockBasedCompensation</Role>
      <ShortName>Stock-Based Compensation</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>16</Position>
    </Report>
    <Report instance="ctmx-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R17.htm</HtmlFileName>
      <LongName>995555 - Disclosure - Leases</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20260630/taxonomy/role/DisclosureLeases</Role>
      <ShortName>Leases</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>17</Position>
    </Report>
    <Report instance="ctmx-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R18.htm</HtmlFileName>
      <LongName>995565 - Disclosure - Income Taxes</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20260630/taxonomy/role/DisclosureIncomeTaxes1</Role>
      <ShortName>Income Taxes</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>18</Position>
    </Report>
    <Report instance="ctmx-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R19.htm</HtmlFileName>
      <LongName>995575 - Disclosure - Segment Disclosures</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20260630/taxonomy/role/DisclosureSegmentDisclosures</Role>
      <ShortName>Segment Disclosures</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>19</Position>
    </Report>
    <Report instance="ctmx-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R20.htm</HtmlFileName>
      <LongName>995585 - Disclosure - Basis of Presentation and Summary of Significant Accounting Policies (Policies)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20260630/taxonomy/role/Role_DisclosureBasisOfPresentationAndSummaryOfSignificantAccountingPoliciesPolicies</Role>
      <ShortName>Basis of Presentation and Summary of Significant Accounting Policies (Policies)</ShortName>
      <MenuCategory>Policies</MenuCategory>
      <ParentRole>http://cytomx.com/20260630/taxonomy/role/Role_DisclosureBasisOfPresentationAndSummaryOfSignificantAccountingPolicies</ParentRole>
      <Position>20</Position>
    </Report>
    <Report instance="ctmx-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R21.htm</HtmlFileName>
      <LongName>995595 - Disclosure - Net Income (Loss) Per Share (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20260630/taxonomy/role/DisclosureNetIncomeLossPerShareTables</Role>
      <ShortName>Net Income (Loss) Per Share (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://cytomx.com/20260630/taxonomy/role/DisclosureNetIncomeLossPerShare</ParentRole>
      <Position>21</Position>
    </Report>
    <Report instance="ctmx-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R22.htm</HtmlFileName>
      <LongName>995605 - Disclosure - Fair Value Measurements and Investments (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20260630/taxonomy/role/DisclosureFairValueMeasurementsAndInvestmentsTables</Role>
      <ShortName>Fair Value Measurements and Investments (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://cytomx.com/20260630/taxonomy/role/DisclosureFairValueMeasurementsAndInvestments</ParentRole>
      <Position>22</Position>
    </Report>
    <Report instance="ctmx-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R23.htm</HtmlFileName>
      <LongName>995615 - Disclosure - Accrued Liabilities (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20260630/taxonomy/role/Role_DisclosureAccruedLiabilitiesTables</Role>
      <ShortName>Accrued Liabilities (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://cytomx.com/20260630/taxonomy/role/Role_DisclosureAccruedLiabilities</ParentRole>
      <Position>23</Position>
    </Report>
    <Report instance="ctmx-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R24.htm</HtmlFileName>
      <LongName>995625 - Disclosure - Collaboration and License Agreements (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20260630/taxonomy/role/DisclosureCollaborationAndLicenseAgreementsTables</Role>
      <ShortName>Collaboration and License Agreements (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://cytomx.com/20260630/taxonomy/role/DisclosureCollaborationAndLicenseAgreements</ParentRole>
      <Position>24</Position>
    </Report>
    <Report instance="ctmx-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R25.htm</HtmlFileName>
      <LongName>995635 - Disclosure - Common Stock (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20260630/taxonomy/role/DisclosureCommonStockTables</Role>
      <ShortName>Common Stock (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://cytomx.com/20260630/taxonomy/role/Role_DisclosureCommonStock</ParentRole>
      <Position>25</Position>
    </Report>
    <Report instance="ctmx-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R26.htm</HtmlFileName>
      <LongName>995645 - Disclosure - Stock-Based Compensation (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20260630/taxonomy/role/Role_DisclosureStockBasedCompensationTables</Role>
      <ShortName>Stock-Based Compensation (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://cytomx.com/20260630/taxonomy/role/Role_DisclosureStockBasedCompensation</ParentRole>
      <Position>26</Position>
    </Report>
    <Report instance="ctmx-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R27.htm</HtmlFileName>
      <LongName>995655 - Disclosure - Leases (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20260630/taxonomy/role/DisclosureLeasesTables</Role>
      <ShortName>Leases (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://cytomx.com/20260630/taxonomy/role/DisclosureLeases</ParentRole>
      <Position>27</Position>
    </Report>
    <Report instance="ctmx-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R28.htm</HtmlFileName>
      <LongName>995665 - Disclosure - Segment Disclosures (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20260630/taxonomy/role/DisclosureSegmentDisclosuresTables</Role>
      <ShortName>Segment Disclosures (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://cytomx.com/20260630/taxonomy/role/DisclosureSegmentDisclosures</ParentRole>
      <Position>28</Position>
    </Report>
    <Report instance="ctmx-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R29.htm</HtmlFileName>
      <LongName>995675 - Disclosure - Net Income (Loss) Per Share - Summary of Calculation of Basic and Diluted Net Income Per Share (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20260630/taxonomy/role/DisclosureNetIncomeLossPerShareSummaryOfCalculationOfBasicAndDilutedNetIncomePerShareDetails</Role>
      <ShortName>Net Income (Loss) Per Share - Summary of Calculation of Basic and Diluted Net Income Per Share (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://cytomx.com/20260630/taxonomy/role/DisclosureNetIncomeLossPerShareTables</ParentRole>
      <Position>29</Position>
    </Report>
    <Report instance="ctmx-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R30.htm</HtmlFileName>
      <LongName>995685 - Disclosure - Net Income (Loss) Per Share - Summary of Potentially Dilutive Securities Excluded from Computation of Diluted Net Income (Loss) Per Share (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20260630/taxonomy/role/DisclosureNetIncomeLossPerShareSummaryOfPotentiallyDilutiveSecuritiesExcludedFromComputationOfDilutedNetIncomeLossPerShareDetails</Role>
      <ShortName>Net Income (Loss) Per Share - Summary of Potentially Dilutive Securities Excluded from Computation of Diluted Net Income (Loss) Per Share (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://cytomx.com/20260630/taxonomy/role/DisclosureNetIncomeLossPerShareTables</ParentRole>
      <Position>30</Position>
    </Report>
    <Report instance="ctmx-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R31.htm</HtmlFileName>
      <LongName>995695 - Disclosure - Fair Value Measurements and Investments - Schedule of Investments Subject to Fair Value Measurements on a Recurring Basis (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20260630/taxonomy/role/DisclosureFairValueMeasurementsAndInvestmentsScheduleOfInvestmentsSubjectToFairValueMeasurementsOnARecurringBasisDetails</Role>
      <ShortName>Fair Value Measurements and Investments - Schedule of Investments Subject to Fair Value Measurements on a Recurring Basis (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>31</Position>
    </Report>
    <Report instance="ctmx-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R32.htm</HtmlFileName>
      <LongName>995705 - Disclosure - Fair Value Measurements and Investments - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20260630/taxonomy/role/DisclosureFairValueMeasurementsAndInvestmentsAdditionalInformationDetails</Role>
      <ShortName>Fair Value Measurements and Investments - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>32</Position>
    </Report>
    <Report instance="ctmx-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R33.htm</HtmlFileName>
      <LongName>995715 - Disclosure - Accrued Liabilities - Summary of Accrued Liabilities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20260630/taxonomy/role/Role_DisclosureAccruedLiabilitiesSummaryOfAccruedLiabilitiesDetails</Role>
      <ShortName>Accrued Liabilities - Summary of Accrued Liabilities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>33</Position>
    </Report>
    <Report instance="ctmx-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R34.htm</HtmlFileName>
      <LongName>995725 - Disclosure - Collaboration and License Agreements - Schedule of Revenue by Collaboration Partners (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20260630/taxonomy/role/DisclosureCollaborationAndLicenseAgreementsScheduleOfRevenueByCollaborationPartnersDetails</Role>
      <ShortName>Collaboration and License Agreements - Schedule of Revenue by Collaboration Partners (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>34</Position>
    </Report>
    <Report instance="ctmx-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R35.htm</HtmlFileName>
      <LongName>995735 - Disclosure - Collaboration and License Agreements - Amgen, Inc - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20260630/taxonomy/role/DisclosureCollaborationAndLicenseAgreementsAmgenIncAdditionalInformationDetails</Role>
      <ShortName>Collaboration and License Agreements - Amgen, Inc - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>35</Position>
    </Report>
    <Report instance="ctmx-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R36.htm</HtmlFileName>
      <LongName>995745 - Disclosure - Collaboration and License Agreements - Astellas Pharma Inc - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20260630/taxonomy/role/DisclosureCollaborationAndLicenseAgreementsAstellasPharmaIncAdditionalInformationDetails</Role>
      <ShortName>Collaboration and License Agreements - Astellas Pharma Inc - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>36</Position>
    </Report>
    <Report instance="ctmx-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R37.htm</HtmlFileName>
      <LongName>995755 - Disclosure - Collaboration and License Agreements - Bristol-Myers Squibb Company - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20260630/taxonomy/role/DisclosureCollaborationAndLicenseAgreementsBristolmyersSquibbCompanyAdditionalInformationDetails</Role>
      <ShortName>Collaboration and License Agreements - Bristol-Myers Squibb Company - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>37</Position>
    </Report>
    <Report instance="ctmx-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R38.htm</HtmlFileName>
      <LongName>995765 - Disclosure - Collaboration and License Agreements - ModernaTX, Inc - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20260630/taxonomy/role/DisclosureCollaborationAndLicenseAgreementsModernatxIncAdditionalInformationDetails</Role>
      <ShortName>Collaboration and License Agreements - ModernaTX, Inc - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>38</Position>
    </Report>
    <Report instance="ctmx-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R39.htm</HtmlFileName>
      <LongName>995775 - Disclosure - Collaboration and License Agreements - Regeneron Pharmaceuticals, Inc - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20260630/taxonomy/role/DisclosureCollaborationAndLicenseAgreementsRegeneronPharmaceuticalsIncAdditionalInformationDetails</Role>
      <ShortName>Collaboration and License Agreements - Regeneron Pharmaceuticals, Inc - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>39</Position>
    </Report>
    <Report instance="ctmx-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R40.htm</HtmlFileName>
      <LongName>995785 - Disclosure - Collaboration and License Agreements - Summary of Contract Liabilities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20260630/taxonomy/role/DisclosureCollaborationAndLicenseAgreementsSummaryOfContractLiabilitiesDetails</Role>
      <ShortName>Collaboration and License Agreements - Summary of Contract Liabilities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>40</Position>
    </Report>
    <Report instance="ctmx-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R41.htm</HtmlFileName>
      <LongName>995795 - Disclosure - Collaboration and License Agreements - Contract Liabilities - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20260630/taxonomy/role/DisclosureCollaborationAndLicenseAgreementsContractLiabilitiesAdditionalInformationDetails</Role>
      <ShortName>Collaboration and License Agreements - Contract Liabilities - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>41</Position>
    </Report>
    <Report instance="ctmx-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R42.htm</HtmlFileName>
      <LongName>995805 - Disclosure - License Agreement - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20260630/taxonomy/role/DisclosureLicenseAgreementAdditionalInformationDetails</Role>
      <ShortName>License Agreement - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>42</Position>
    </Report>
    <Report instance="ctmx-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R43.htm</HtmlFileName>
      <LongName>995815 - Disclosure - Common Stock - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20260630/taxonomy/role/DisclosureCommonStockAdditionalInformationDetails</Role>
      <ShortName>Common Stock - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>43</Position>
    </Report>
    <Report instance="ctmx-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R44.htm</HtmlFileName>
      <LongName>995825 - Disclosure - Common Stock - Summary of Company's Warrants (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20260630/taxonomy/role/DisclosureCommonStockSummaryOfCompanysWarrantsDetails</Role>
      <ShortName>Common Stock - Summary of Company's Warrants (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>44</Position>
    </Report>
    <Report instance="ctmx-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R45.htm</HtmlFileName>
      <LongName>995835 - Disclosure - Stock-Based Compensation - Schedule of Activities Under Company's Stock Option Plans (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20260630/taxonomy/role/Role_DisclosureStockBasedCompensationScheduleOfActivitiesUnderCompanySStockOptionPlansDetails</Role>
      <ShortName>Stock-Based Compensation - Schedule of Activities Under Company's Stock Option Plans (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>45</Position>
    </Report>
    <Report instance="ctmx-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R46.htm</HtmlFileName>
      <LongName>995845 - Disclosure - Stock-Based Compensation - Schedule of Company's TRSU Activities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20260630/taxonomy/role/DisclosureStockbasedCompensationScheduleOfCompanysTrsuActivitiesDetails</Role>
      <ShortName>Stock-Based Compensation - Schedule of Company's TRSU Activities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>46</Position>
    </Report>
    <Report instance="ctmx-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R47.htm</HtmlFileName>
      <LongName>995855 - Disclosure - Stock-Based Compensation - Schedule of Activities for Company's PSUs (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20260630/taxonomy/role/DisclosureStockbasedCompensationScheduleOfActivitiesForCompanysPsusDetails</Role>
      <ShortName>Stock-Based Compensation - Schedule of Activities for Company's PSUs (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>47</Position>
    </Report>
    <Report instance="ctmx-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R48.htm</HtmlFileName>
      <LongName>995865 - Disclosure - Stock-Based Compensation - Total Stock-based Compensation Recognized (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20260630/taxonomy/role/Role_DisclosureStockBasedCompensationTotalStockBasedCompensationRecognizedDetails</Role>
      <ShortName>Stock-Based Compensation - Total Stock-based Compensation Recognized (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>48</Position>
    </Report>
    <Report instance="ctmx-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R49.htm</HtmlFileName>
      <LongName>995875 - Disclosure - Stock-Based Compensation - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20260630/taxonomy/role/DisclosureStockbasedCompensationAdditionalInformationDetails</Role>
      <ShortName>Stock-Based Compensation - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>49</Position>
    </Report>
    <Report instance="ctmx-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R50.htm</HtmlFileName>
      <LongName>995885 - Disclosure - Leases - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20260630/taxonomy/role/DisclosureLeasesAdditionalInformationDetails</Role>
      <ShortName>Leases - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>50</Position>
    </Report>
    <Report instance="ctmx-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R51.htm</HtmlFileName>
      <LongName>995895 - Disclosure - Leases - Summary of Supplemental Information Related to Leases (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20260630/taxonomy/role/DisclosureLeasesSummaryOfSupplementalInformationRelatedToLeasesDetails</Role>
      <ShortName>Leases - Summary of Supplemental Information Related to Leases (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>51</Position>
    </Report>
    <Report instance="ctmx-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R52.htm</HtmlFileName>
      <LongName>995905 - Disclosure - Leases - Schedule of Undiscounted Future Non-Cancelable Lease Payments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20260630/taxonomy/role/DisclosureLeasesScheduleOfUndiscountedFutureNoncancelableLeasePaymentsDetails</Role>
      <ShortName>Leases - Schedule of Undiscounted Future Non-Cancelable Lease Payments (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>52</Position>
    </Report>
    <Report instance="ctmx-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R53.htm</HtmlFileName>
      <LongName>995925 - Disclosure - Income Taxes - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20260630/taxonomy/role/DisclosureIncomeTaxesAdditionalInformationDetails</Role>
      <ShortName>Income Taxes - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>53</Position>
    </Report>
    <Report instance="ctmx-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R54.htm</HtmlFileName>
      <LongName>995935 - Disclosure - Segment Disclosures - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20260630/taxonomy/role/DisclosureSegmentDisclosuresAdditionalInformationDetails</Role>
      <ShortName>Segment Disclosures - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>54</Position>
    </Report>
    <Report instance="ctmx-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R55.htm</HtmlFileName>
      <LongName>995945 - Disclosure - Segment Disclosures - Schedule of Revenue and Expenses (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20260630/taxonomy/role/DisclosureSegmentDisclosuresScheduleOfRevenueAndExpensesDetails</Role>
      <ShortName>Segment Disclosures - Schedule of Revenue and Expenses (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>55</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <LongName>All Reports</LongName>
      <ReportType>Book</ReportType>
      <ShortName>All Reports</ShortName>
    </Report>
  </MyReports>
  <InputFiles>
    <File doctype="10-Q" isDefinitelyFs="true" isUsgaap="true" original="ctmx-20260630.htm">ctmx-20260630.htm</File>
    <File>ctmx-20260630.xsd</File>
  </InputFiles>
  <SupplementalFiles/>
  <BaseTaxonomies>
    <BaseTaxonomy items="585">http://fasb.org/us-gaap/2025</BaseTaxonomy>
    <BaseTaxonomy items="30">http://xbrl.sec.gov/dei/2025</BaseTaxonomy>
    <BaseTaxonomy items="4">http://xbrl.sec.gov/ecd/2025</BaseTaxonomy>
  </BaseTaxonomies>
  <HasPresentationLinkbase>false</HasPresentationLinkbase>
  <HasCalculationLinkbase>false</HasCalculationLinkbase>
</FilingSummary>
